Fluid system having controllable reversible viscosity

ABSTRACT

This Invention relates to methods of treating a subterranean hydrocarbons reservoir comprising contacting the formation with a treating fluid comprising an aqueous solution, an acid, a surfactant acting as gelling agent essentially consisting of erucylamidopropyl betaine (or a protonated/deprotonated homolog or salt thereof). The treating fluid may further comprise a lower n-alcohol for improved temperature stability.

This application is a continuation of application Ser. No. 10/065,144filed Sep. 20, 2002 now U.S. Pat. No. 7,119,050, which claims thebenefit of provisional Application No. 60/343,145 filed Dec. 21, 2001.

STATEMENT OF COOPERATIVE RESEARCH AGREEMENT

The present invention, as defined by the claims herein, was made byparties to a Joint Research Agreement between Schlumberger TechnologyCorporation and Rhodia, Inc., as a result of activities undertakenwithin the scope of that Agreement relating at least to the field of theinvention as described below. The Agreement was in effect prior to thedate of the invention, and at the time the invention was made.

TECHNICAL FIELD OF THE INVENTION

This invention relates to fluid compositions whose viscosity can becarefully modulated—from very low viscosity to sufficient viscosity toact as a barrier to further flow. More specifically, the inventionrelates to diverting fluids used for stimulating hydrocarbon-bearingformations—i.e., to increase the production of oil/gas from theformation.

INTRODUCTION TO THE TECHNOLOGY

Hydrocarbons (oil, natural gas, etc.) are obtained from a subterraneangeologic formation (i.e., a “reservoir”) by drilling a well thatpenetrates the hydrocarbon-bearing formation and thus causing a pressuregradient that forces the fluid to flow from the reservoir to the well.Often, a well production is limited by poor permeability either due tonaturally tight formations or to formation damages typically arisingfrom prior well treatment, such as drilling, cleaning etc.

To increase the net permeability of a reservoir, it is common to performa well stimulation. The most common stimulation technique consists ininjecting an acid that reacts with and dissolve the damage or portion ofthe formation thereby creating alternative flowpaths for thehydrocarbons to migrate through the formation to the well. Thistechnique known as acidizing may eventually be associated withfracturing if the injection rate and pressure is enough to induce theformation of a fracture in the reservoir.

Fluid placement is critical to the success of stimulation treatment.Natural reservoirs are often heterogeneous; the acid fluid willpreferentially enter areas of higher permeability in lieu of treatingthe areas where a treatment is most needed. Similarly, acid treatmenttends to remove the damages that are easier to reach, due to a lowerdegree of damage or higher permeability. Each additional volume of acidfluid follows the path of less resistance, failing to remove the mostimportant damages. The critical character of fluid placement isexacerbated by the fact that the acid reacts extremely quickly andconsequently, is spent almost immediately and therefore not availablefor treating the untreated areas.

THE PRIOR ART

In order to control placement of treating fluids, various techniqueshave been employed. Mechanical techniques involve for instance the useof ball sealers and packers and of coiled tubing placement tospecifically spot the fluid across the zone of interest. Non mechanicaltechniques typically make use of gelling agents as diverters fortemporary impairing the areas of higher permeability and increasing theproportion of the treating zone that goes into the areas of lowerpermeability. Of course, a diverter should not itself damage thereservoir and therefore it is important that it can be easily removedfollowing the acid treatment so that the zones of higher permeabilityremain so.

Most commercialized chemical diverters are based on cross-linkedpolymers. Unfortunately, these systems leave a residue in the formation,which can damage the formation, resulting in diminished hydrocarbonproduction. In addition, the cross-linking reaction is easily perturbedby formation chemistry, contaminants in the pumping equipment, and soforth.

It is also known to use self-diverting acids, typically consisting ofhydrochloric acid mixed with a gelling agent and a pH-sensitivecross-linker. Self-diverting acids are typically designed to gel atintermediate pH values, when the acid is partially spent. Self-divertingsystems not based on cross-linking chemistry by which rely uponviscoelastic surfactants are described in U.S. Pat. No. 4,695,389 (seealso, U.S. Pat. No. 4,324,669, and British Patent No. 2,012,830, bothcited there)-which has a common assignee as the present application.Viscoelastic surfactants based systems exhibit very low frictionpressure and therefore are easy to pump and yet, form a gel downhole.U.S. Pat. No. 4,695,389 discloses a viscoelastic surfactant-basedgelling agent intended for use in acid fracturing. The particularlypreferred embodiment is a fluid comprised of N,N-bis(2-hydroxyethyl)fatty amine acetic acid salt (the gelling agent), an alkali metalacetate salt, acetic acid (the acid-which actually removes the damagefrom the formation), and water.

Improved self-diverting systems have been described in U.S. patentapplication Ser. No. 09/419,842, having a common assignee as the presentapplication and its corresponding International Patent Application WO01/29369. This application, hereby incorporated by reference, providesformulations suitable for acid treatment comprising an amphotericsurfactant that gels as the acid spends in presence of an activatingamount of a co-surfactant and of multivalent cations typically generatedby the acid reaction with the formation. When the gelling agent is mixedin hydrochloric acid, the co-surfactant prevents the gelling of thesolution; the solution gels when the pH increases above 2.

In a preferred embodiment, the amphoteric surfactant is oleylpropylbetaine of the formula:

and the co-surfactant is preferably sodium dodecyl benzene sulfonate.

Compositions known from the U.S. patent application Ser. No. 09/419,842,and its corresponding International Patent Application WO 01/29369, havebeen of limited commercial applications by due to temperaturelimitations. In particular, formulations based on oleylamidopropylbetaine and sodium dodecyl benzene sulfonate can only be used fortemperature below about 200° F. (93° C.) while the carbonate acidizingmarket calls for temperature up to about 300° F. (about 150° C.).

It is therefore an object of the present invention to provide newself-diverting acid formulations with improved temperature stability.

SUMMARY OF THE INVENTION

According to a first embodiment of the present invention, there isprovided a method for stimulating a subterranean hydrocarbons reservoircomprising contacting the formation with a treating fluid consisting ofan acid, erucylamidopropyl betaine (or a protonated, deprotonated,homolog or salt thereof), and an-alcohol, typically selected amongmethanol and/or ethanol, and most preferably being methanol. Theformulation of the present invention may also comprise various standardadditives such as corrosion inhibitors, non-emulsifiers and iron controlagents. One embodiment is a method of treating a subterranean formationinvolving contacting the formation with a treating fluid containing anaqueous solution, an acid selected from an organic acid and an inorganicacid, and a surfactant acting as gelling agent essentially consisting oferucylamidopropyl betaine, or a protonated/deprotonated homolog or saltthereof, without a co-surfactant.

As for the compositions known from U.S. patent application Ser. No.09/419,842, the compositions of the present invention exhibit a nearlywater-like viscosity upon addition of the acid to the base fluidconsisting of water and the viscoelastic surfactant (theerucylamidopropyl betaine). A viscous gel starts to develop when theacid spends to react with a carbonate formation. The gel maintains ahigh viscosity so continuous diversion can be achieved during pumping.After completion of the job and resume of the hydrocarbon production,the produced hydrocarbons lower the gel viscosity down to nearwater-like viscosity, leaving no residue.

Most preferably the compositions of the present invention comprisemethanol, preferably at concentration of between about 1% and about 10%,by volume, most preferably at concentration of between about 5 and 10%by volume. Methanol essentially performs two functions: it improves thegel viscosity of the spent acid at high temperature and prevents thegelling of unspent solution of moderate acid strength (below 20wt % ofhydrochloric acid). Consequently, the system of the present inventiondoes not require the addition of a co-surfactant. With higher acidstrength, the unspent solution does not gel so that the addition ofalcohol is optional depending on the temperature range of theapplication. In any case, it has been found that with erucylamidopropylbetaine, no cosurfactant is required for the gel formation.

The acid is selected from the group consisting of hydrochloric, amixture of hydrochloric and hydrofluoric acids, fluoroboric acid, nitricacid, phosphoric acid, maleic acid, citric acid, acetic acid, and formicacid. According to a preferred embodiment of the invention, the acid ishydrochloric acid and is added at a concentration of between about 3%and about 28% by weight, most typically at concentration between about15% and about 28%.

According to a preferred embodiment of the invention, the viscoelasticsurfactant is added at a concentration of between about 1 and about 4%,by weight active material (the surfactant is typically delivered insolution). Most preferably, the surfactant is added at activeconcentration between about 2 and about 3% by weight. Higherconcentrations may be used depending on the formation permeability rangeand contrast.

In a most preferred embodiment, for applications where the bottomholestatic temperature ranges from about 25° C. to about 150° C., aformulation comprising 3 weight percent of viscoelastic surfactant and10 volume percent % of methanol is recommended.

Unlike other diverting materials, the fluids of the invention can bepumped as a single fluid, which will stimulate and divert in one step.It can be bullheaded down tubing or, according to a preferredembodiment, placed using coiled tubing moved up while injecting theacidic formulation. According to another embodiment, the fluid is pumpedin several stages, alternately with regular acid stages.

As it is the rule for acid treatment, the formulation will typicallycomprise corrosion inhibitors, most preferably based on quaternaryamines. Further agents may be typically added such as for instancenon-emulsifier and iron reducing agent, chelating agent. It should benoted that the formulation of the present invention is sensitive toiron, in particular to ferric ions. A preflush treatment with ironreducing agent and chelating agent is therefore recommended before theacid treatment. Though the formulation of the invention is compatiblewith small concentration of non-emulsifying agent, to prevent emulsionand sludge, it is also a good practice to preflush the well with amutual solvent, preferably low molecular weight esters, ether andalcohols, and more preferably ethylene glycol monobutyl ether.

Though the present Invention is directed primarily to matrix acidizing,it is entirely applicable to a closely related stimulation technique,acid fracturing, which is very similar, but involves pumping the acid ator above pressures sufficient to fracture the formation (minimum in siturock stress). For convenience sake, the focus here shall be directed tomatrix acidizing.

BRIEF DESCRIPTION OF DRAWINGS

The above and further objects, features and advantages of the presentinvention will be better understood by reference to the appendeddetailed description and to the drawings wherein:

FIG. 1 shows the viscosity profile of solutions comprisingerucylamidopropyl betaine as a function of pH

FIG. 2 shows the viscosity profile at a shear rate of 40 sec−¹, of twogels, with or without added methanol, at temperature ranging from roomtemperature to about 150° C.;

FIG. 3 shows the viscosity profile of gels, with or without addedmethanol, as a function of temperature and shear rate;

FIG. 4 shows the viscosity profile of various gels with variousconcentration of methanol, as a function of temperature;

FIG. 5 shows comparative viscosity profiles of various gels with noalcohol, methanol and ethanol for a gel comprising 28% HCl;

FIG. 6 shows comparative viscosity profiles of various gels with noalcohol, methanol and ethanol for a gel comprising 15% HCl;

The composition of the present invention are erucylamidopropyl betaine,an amphoteric surfactant having the general formula:

The viscosity development of a solution comprising erucylamidopropylbetaine during acid spent is illustrated FIG. 1. This test was performedat room temperature, with a solution comprising 15% HCl and 3 wt % oferucylamidopropyl betaine. The viscosity was measured at a shear rate of170 s⁻¹. As soon as the spending starts, the viscosity increases. To benoted that the gel is broken by the hydrocarbons of the subterraneanformation.

FIG. 2 shows the viscosity (using a logarithm scale) of a gel atelevated temperature. The gel was prepared by adding first 0.6% of acorrosion inhibitor and 2% of corrosion inhibitor aid into a 20% HClsolution with or without methanol. The surfactant is then added to thesolution at an active concentration of 3% by weight. After welldispersed, the mixture was then let react with powder CaCO₃ until theacid is neutralized to a pH value of 3.5 to 4 at 180° F. A viscoelasticgel forms when the pH reaches above 2 to 2.5. The gel is loaded in aFann 50 rheometer to measure the viscosity (in CentiPoises ormilliPascals second) at specified temperatures, at a shear rate of 40sec⁻¹.

The gel that does not contain methanol has a viscosity profilerepresented by a dotted line. The gel reaches a peak of viscosity for atemperature ranging between 120° F. and 130° F. (about 50° C.). Athigher temperature, the viscosity sharply declines, followed by alowered plateau near 100 cP from 150° F. to 200° F. (about 65° C. toabout 93° C.). The addition of methanol (plain line) is detrimental tothe quality of the gel at lower temperatures but helps extends thetemperature at which the gel exhibits the maximum viscosity.

In FIG. 3, the viscosity of the gel at different shear rates (40, 100,and 170 sec⁻¹) is presented. The gels are prepared the same way asdescribed in the example of FIG. 2, except for the surfactantconcentration and the starting HCl concentration. In this currentexample the HCl concentration used is 28% while in the previous examplethe acid concentration is 20% before spending. The BET-E surfactant isthen added to the solution at an active concentration of 3% by weight.

To better reflect the stress at which the gel is exposed during the welltreatment, the rheology is represented by a cluster of curvescorresponding to different shear rates (open marks for the gels withoutmethanol, filled marks with; viscosity at shear rates respectively equalto 40, 70 and 170 are represented respectively by circles, squares andtriangles. For the gel without methanol, the curves show a flatviscosity from 75° F. to 175° F., then the viscosity sharply declines inthe temperature from 175° F. to 225° F., followed by a lowered plateaufrom 225° F. to 300° F. The second cluster of curves represents the gelmade with 10% by volume methanol. The viscosity increases from 75° F. to200° F. and maintained a plateau until 250° F., then a gentle declinetoward 300° F.

FIG. 4 shows the viscosity of gels similar to the gels of the previousFIG. 3, thus comprising 3 w % of surfactant and 28% HCl, with variousvolume concentration of methanol. The four tested compositions compriserespectively no methanol (stars), 1% (open circles), 6% (filledtriangles) and 10% (filled squares). An addition of only 1% of methanolis enough to significantly improve the viscosity at temperatures rangingbetween about 200° F. and 250° F. (about 93° C. to about 120° C.). Withhigher concentrations of methanol, the temperature limit can be extendedup to about 300° F. (about 150° C.). For the tested formulation, bestresults were obtained with compositions comprising between about 6% and8% of methanol. Higher methanol concentrations lead to a decrease of thegel viscosity for lower temperature.

FIG. 5 shows that ethanol also helps to extend the maximum temperatureof use of the gel but to a much smaller extent. FIG. 5 presents resultsobtained with gels similar to the ones of FIGS. 3 and 4. The dotted linecorresponds to a gel without alcohol; the open squares to a gel with 1%of methanol, the filled triangles to a gel with 1% of ethanol.

FIG. 6 shows similar results obtained with a solution comprising only15% HCl. Ethanol does not prove to be effective. Tests have shown thatin 15% HCl, a solution comprising 5% of ethanol does not gel when acidspends.

1. A method of treating a subterranean formation comprising contactingthe formation with a treating fluid comprising an aqueous solution, anacid, methanol at a concentration of between 1 and 10% by volume, and asurfactant acting as gelling agent essentially consisting oferucylamidopropyl betaine or a protonated/deprotonated homolog or saltthereof.
 2. The method of claim 1 wherein said acid is selected from thegroup consisting of fluoroboric acid, nitric acid, phosphoric acid,maleic acid, and citric acid.
 3. The method of claim 1, wherein saidacid is present in said fluid at a concentration of from about 15% byweight to about 28% by weight.
 4. The method of claim 1 wherein saidacid is selected from the group consisting of hydrochloric acid, amixture of hydrofluoric acid and hydrochloric acid, acetic acid andformic acid.